PCB Radiated Emissions With Linked Enclosure

PCB Radiated
Emissions Linked with
Enclosure
Dane Thompson
Application Engineer
Ansoft

PCB Radiated Emissions
Linked with Enclosure
A three step design flow to
improve EMI/EMC performance

� Defining the Problem
Outline
� The terms Electromagnetic Compatibility (EMC) and
Electromagnetric Interference (EMI) have grown in meaning
over the years. The scope of the problem to be solved will
first be defined.
� Applying Numerical Analysis to EMI/EMC
� Describing the multi-scale problem for EMI/EMC
� Three-step design approach for EMI/EMC
� PCB in an Enclosure
� Ground to Shield Coupling

What does EMI/EMC mean?
� Undesired signals interfere with system performance.
� Examples:
� Car radio interference due to automobile electronics.
� Mobile phone interference with television, radio, etc.
� Cross-talk between digital and analog signals on a PCB
resulting in reduced degraded system performance.
� Noise on power and ground planes due to switching.
� The mechanisms responsible for these problems are
due to electromagnetic effects.

EMI and EMC Standards
� A formal definition of EMI/EMC….
IEEE Project 1597.1 „IEEE Standard for Validation of Computational
Electromagnetics (CEM) Computer Modeling and Simulation“
IEEE Project 1597.2 „IEEE Recommended Practice for Computational
Electromagnetics (CEM) Computer Modeling and Simulation Applications“
European Union Directive 1999/5/EC
European Union Directive 2004/108/EC
Electromagnetic disturbance means any electromagnetic phenomenon
which may degrade the performance of equipment. An electromagnetic
disturbance may be electromagnetic noise, an unwanted signal, or a change
in the propagation medium iteslf.
Immunity means the ability of equipment to perform as intended without
degradation in the presence of an electromagnetic disturbance.
See also: http://aces.ee.olemiss.edu/ Public domain benchmarks for EMI/EMC Simulation
http://www.ewh.ieee.org/soc/emcs/ IEEE EMC Society

Approaches to using Electromagnetic
Simulation in an EMI/EMC Design Flow
� A major EMI/EMC Problem for complex printed circuit boards is
emission of signals on the PCB.
� The complexity of such problems is enormous (this is known as
the multi-scale problem):
One section of a multi-layer
PCB that may radiate.

Overcoming the Multi-Scale Problem in
EMI/EMC Analysis
� The multi-scale problem means that a single electromagnetic
analysis will most likely not address the system solution.
� The printed circuit board is comprised of a large number of nets, as
well as many active and passive components, all of which play a
role in electromagnetic emission.
� The smallest feature sizes are on the order of several microns, while
the largest features may be 10‘s of centimeters.
Solution! Solution?
almost
Radiation leakage from the PCB will almost always
be minimized using shielding.
… thus, the coupling between the
PCB and 3D analysis is needed.
+

3-Step Design Approach for EMC
1. Identify critical components in the system and
analyze them individually using HFSS or Q3D.
� Traces supporting high speed data,
� Eigenmode analysis to identify resonances.
� Other packages, comonents, antennas.
2. Signal and Power Integrity Analysis in Nexxim and
SIwave
� Analysis in SIwave combined with circuit analysis can be
used to verify circuit performance and identify signal integrity
issues.
3. Combine PCB extraction, circuit analysis and 3D
electromagnetic analysis to verify EMC system
performance.

3-Step Design Approach (Step 1)
� Examine the properties of the isolated EMC shield:
Outer shield wall
(shown as wireframe)
PCB
HFSS Model
Lumped source
excitation
Inner shield wall
� Low frequency resonances may occur in shielding due to long
current paths arising from gaps in the shielding.

Invesitgating the EMC Shield in
HFSS
� Coupling to a resonance can be seen by the drop-out in S11 at
4.09 GHz, indicating a possible EMI problem.
S11 (dB)

Investigating the EMC shield in HFSS
� The Eigenmode solver in HFSS can be used to find the resonance
as well.
E-field magnitude of the driven
problem at 4.09 GHz
E-field magnitude of the Eigenmode
resonance at 3.97 GHz
� Eigenmode analysis adapts the mesh to the field distribution at
resonance.
� Radiation boundary cannot be used in the eigenmode solver, so the
resonance will be slightly different than that determined by the driven
solution.

3-Step Design Approach (Step 2)
� Prepare PCB for simulation
� Use Ansoft Links to import the layout
and components from the 3rd party
layout program
Design export from Cadence,
Mentor, Synopsis, Zuken tools
Ansoft Links
Clean-up, choose critical nets, de-feature
and export 3D model (HFSS / Q3D
analysis)
SIwave
Analysis of
complete PCB

3-Step Design Approach (Step 2)
� The three-step approach for EMI/EMC is directly
linked to the signal and power integrity design-flow.
Import ANF imports layout,
stackup and padstack
information for full-wave
analysis.
Import CMP imports
component information
including component type,
location and value.

After full-wave analysis, a
Full-Wave SPICE model of
the PCB can be generated.
Analysis in SIwave
PCB Model in SIwave
IC‘s as place-holders for
circuit analysis in NEXXIM
Resistors, Capacitors and Inductors are
included in the SIwave model.

3-Step Design Approach (Step 2)
Ibis drivers
� Signal integrity analysis can be carried out by combining circuit and
system level analyses.
Passive comonents
Voltage supply

Circuit Analysis
EMC Analysis from PCB
Nexxim
Complex spectral data from SPICE
analysis can be loaded into SIwave.
SIwave
Full-wave PCB
Spectrum from a data
signal.
F (GHz)

Electromagnetic Emission from
the PCB
� The emission spectrum from the PCB exhibits distinct peaks that
correlate with the resonances on the 5V supply.
0.662 GHz
1.047 GHz
2.007 GHz
Normal
Parallel

Examining the plane voltages

Step 3
� The third step in the EMC/EMI design flow aims to verify
the system performance.
� System verification requires the combination of circuit
analysis, PCB full-wave extraction, and 3D
electromagnetic analysis of the shielding and
environment.

3-Step Design Approach (Step 3)
� Use the field distribution in SI-wave as an excitation for an HFSS
project.
The SI-Wave PCB Model is built
into the HFSS simulation.
Bottom shield
0.5 mm gap
HFSS Housing Model

3-Step Design Approach (Step 3)
� The incident field from SI-wave is calculated using the freespace
near-field Green‘s function to project the fields in SIwave
onto a surface in HFSS.
1. A box is placed inside the shield. The fields from SI-wave are
enforced on the surface of this box.
A new „advanced option“ in the radiation
boundary for HFSS Version 10 tells HFSS that
this is the surface on which the fields from SIwave
will be enforced.

3-Step Design Approach (Step 3)
2. Define the Source Excitation in HFSS (this is what is
enforced on the radiation boundary surface).

3-Step Design Approach (Step 3)
3. Link to the SI-wave project. Data is obtained from the frequency
sweep solution.
Far-field in dB(V)
Shielding Resonances
θ component
φ component

3-Step Design Approach (Step 3)
Complex enforced field
distribution from SI-wave
Electric field magnitude with
the SIwave near-field used as
the excitation.
Coupling from the PCB to the shield
resonance results in radiation!
� Eigenmode analysis of this
shield alone predicts a
resonance at 762 MHz.
Eigenmode solution
(no enforced field)

Modified enclosure
� Sheilding that relies on contact
between extended surfaces is
prone to EMI problems because
gaps may form.
� Intentionally designing gaps into the
shielding ensures that the contact
points between two metal shielding
parts are known.
� The spacing between the gaps
should be irregular. Regular
spacing of shielding contact points
leads to a periodic structure which
will be resonant.
Contact points

3-Step Design Approach (Step 3)
3. Housing resonances are suppressed by introducing small contact
points between upper and lower parts of the housing.
Far-field (rE) in dB(V)
0
-20
-40
-60
-80
φ component
θ component
-100
0.5 1.0 1.5 2.0 2.5 3.0

Shielding on a PCB
� The previous example demonstrated EMI for a housing. There was
no direct contact between the shield and PCB.
PCB

Coupling Ground currents to the
� Shield on the PCB
Surface used to enforce
near-field excitation from
the PCB.
shield

Coupling Ground currents to the
� Shield on the PCB
Surface used to enforce
near-field excitation from
the PCB.
shield
Direct contact between the PCB and Shield is made on the edges
of the recess in the plane.

H-field magnitude (current) on
the shield
� The current flows from the PCB into the shield. The currents flowing on the
PCB ground can be determined using circuit analysis and this information can
be used to realistically predict the shield performance.

H-field magnitude
� Viewing the magnetic field we see that coupling to the shield
poses a possible EMI risk.

Linking Nexxim to HFSS
� This is a simple example that demonstrates the effect of
poor grounding.
Signal Input
12V Supply
Ground
Components will go here

Circuit Analysis in Nexxim without parasitics
1Vpp Sinusoid
PNUM=1
RZ=50ohm
IZ=0ohm
R81
10k
R84
DTC11-4EKA U4
10k
T509
R18
10k
R15
10k
T510
0
Spectral Response calculated in
R12
100 Nexxim using Harmonic Balance
T511
BC817-40 U3
BC807-40
U2
0
C11
10nF
0
V19
U1

Circuit Analysis including
parasitic model from HFSS
Spectral Response calculated in
Nexxim using Harmonic Balance

Tangential H-field 3mm above the PCB at
1 MHz

Tangential H-field 3mm above PCB
at 1 MHz

Emission due to nonlinear
harmonics
Spectral Response calculated in
Nexxim using Harmonic Balance
37MHz

Tangential H-field at 37 MHz

H-field at 37 MHz
(due to 1MHz source)

Conclusions
� The multi-scale problem posed by the complex nature of EMI/EMC
design challenges can be addressed using a 3 step approach.
� Compatibility between various electromagnetic simulation techniques
such as circuit analysis (Nexxim), 3-D field analysis (HFSS) and 2-D
finite element analysis (SI-wave) represent a major step toward full
system EMI/EMC electromagnetic analysis.
� Both isolated housing and shielding that is directly connected to the
PCB have been investigated.